1. Field of The Invention
The present invention relates generally to a projection-type television receiver, and more particularly to a receiver, wherein the rays of light of the images formed on the three projection cathode-ray tubes for red, green and blue colors are concentrated through lenses and reflected by mirrors such that said images are projected in registry on a transmissive screen.
2. Description of the Background Art
A conventional type of projection television receiver is shown schematically in FIG. 3. A plan view of the optics used in this TV receiver is shown in FIG. 4, and the relationship between the screen of each of three projection tubes and the images formed thereon is shown in FIG. 5. The principles causing this particular relationship are illustrated in FIGS. 6 and 7. The respective Figures are explained hereinafter.
An array of the three projection tubes is indicated at 1. Positioned at the center is a green projection tube 1G, and a red projection tube 1R and a blue projection tube 1B are arranged on both sides in a row. A lens system is indicated at 2. The lens system includes lenses 2G, 2R and 2B disposed in front of the green projection tube 1G, red projection tube 1R and blue projection tube 1B, respectively. A series of mirrors are indicated by 3a, 3b and 3c. The rays of light of the images formed on the screens of the three projection tubes, 1G, 1R and 1B, are concentrated through respective lenses 2G, 2R and 2B, and reflected three times by mirrors 3a, 3b and 3c in such a manner that the rays are focused on a transmissive screen 5 to display a composite picture of the images from the three projection tubes, 1G, 1R and 1B.
The central projection tube 1G produces a symmetrical image on the transmissive screen 5, since the center line 2Gs of the associated lens 2G is in alignment with the center of the screen 5. Therefore, it suffices that the image 7G is displayed in registry with the center of the screen 6G of the tube 1G.
On the other hand, the right and left projection tubes, 1R and 1B, are disposed offset from the center of the transmissive screen 5. Thus, if the lenses 2R and 2B are disposed such that their respective center lines 2Rs and 2Bs are parallel to the center line 2Gs, the result will be as shown in FIG. 6. In other words, if the images 7R and 7B were displayed as indicated by the solid line in FIG. 6 in such a manner that they agreed with the centers of the screens 6R and 6B of the projection tubes 1R and 1B, the center lines 2Rs and 2Bs would become positioned away from the center of the transmissive screen 5. This would result in each of the images projected from tubes 1R and 1B onto tscreen 5 being deviated from the image projected from 1G, as shown by the solid line in FIG. 6.
In order to avoid this problem, images 7R and 7B formed on the screens 6R and 6B must be offset from their centers, as shown by the dashed line in FIG. 6, such that each of the images projected on the transmissive screen 5 is biased. In this case, the following relationships exist among the focal length f of the lens 2, the distance, a, between the projection tube 1 and the lens 2, the distance, b, between the screen 5 and the lens 2, and the magnification m: EQU 1/a+1/b=1/f and m=b/a.
Each of the images formed on the projection tubes 1R and 1B can be projected onto the transmissive screen 5 to form an image that is in registry with the image from the tube 1G by directing the center lines 2Rs and 2Bs, as shown in FIG. 5, toward the point of intersection between the center line 2Gs and the transmissive screen 5. If this is done, each of the lenses 2R and 2B will be inclined to form an angle .theta. with the transmissive screen 5 as shown in FIG. 7. In order to obtain focus on the front face of the screen 5, the screens 6R and 6B, of the projection tubes 1R and 1B must be inclined by the following angle with respect to the associated lenses, 2R and 2B: 1/m tan .theta.=tan .theta.'. Therefore, if images 7R and 7S were formed on the screens 6R and 6B in alignment with their centers, as shown by the solid line in FIG. 7, the magnifications of the images would become different between the right and left sides. In order to avoid this problem, the images 7R and 7B must be formed in such a manner that they are offset from the centers of the screens 6R and 6B as shown by the dashed line in FIG. 7. At the same time, the difference in magnification must be corrected by processing the images 7R and 7B to a trapezoid form with a method known as "keystone distortion correction", that is commonly employed, as shown in FIG. 5.
This second method is chiefly used in the conventional type of projection television receivers. This method, however, has the disadvantage that an increased amount of power is consumed by a convergence circuit for correctng a keystone distortion, because the screen, 6R (6B), of the projection tube 1R (1B) and the transmissive screen 5 are inclined with respect to the lens 2R (2B) by large angles .theta.' and .theta., respectively. In addition, the images 7R and 7B formed on the screens 6R and 6B are biased from the respective centers, so that the temperature distributions in the screens 6R and 6B (or their faceplates) are not uniform with respect to their centers. This causes a reduction in the reliability of the faceplates, and also fails to make effective use of the fluorescent screen. In order to deal with the latter problem, the magnifying power of the lenses 2R and 2B must be increased. However, this causes the brightness of the transmissive screen 5 to be reduced, since the brightness is in inverse proportion to the square of the magnification (m). Furthermore, the lens 2R (2B) has different angles of view, .beta.1 and .beta.2, with respect to the center lines 2Rs (2Bs), and this makes the brightness of the image projected on the screen 5 different between the right and left sides, since the brightness on the screen 5 is related to cos.sup.4 .beta..
Accordingly, it is an object of the present invention to minimize the occurrence of keystone distortion and thereby reduce power consumption of the convergence circuit in a projection television receiver.
Another object of the invention is to uniformly distribute the temperature on the faceplate of each projection tube with respect to the center of the tube and thereby to improve tube reliability.
It is also an object of the invention to increase the luminance on the transmissive screen. In addition, it is an object of the invention to make the image brightness substantially uniform on both sides (right and left) of the transmissive screen.
Additional objects and advantages will be obvious from the description, or may be learned by practice of the invention.